1. Field of the Invention
This invention relates to distribution transformers and, more particularly, to distribution transformers having high voltage primary disconnect switches to provide isolation of the transformer from the high voltage input circuit.
2. Description of the Prior Art
Oil-filled transformers are used in power distribution systems to transform electrical power from a high voltage suitable for use in transmission to lower voltages suitable for distribution and use within a customer's premises. Repair and maintenance operations often require that the transformer and associated components be isolated from the high voltage transmission circuit serving as input to the transformer. In order to provide this isolation a high voltage disconnect switch is provided. This switch is electrically inserted between the primary coil of the transformer and the high voltage bushing through which the high voltage primary circuit enters the transformer housing. The switch must be constructed so as to minimize the possibility of flashover arcing between components at high potential and other transformer components at ground potential. Thus, the components of the switch must be designed to provide sufficient physical separation between these components.
In dealing with the problem of maintaining sufficient insulation and isolation for switch components, some prior art switches employed plastic operating and support components. Insuring that the dimensions of the components will be held to necessary close tolerances requires extremely accurate molding. In addition, the hot oil environment in which the switch must function is less than ideal for even the best plastic materials. Since metal components having the required tolerances and ability to operate efficiently under hot oil are lower in cost than plastic components meeting similar requirements, it is desirable to provide a switch having metal components which will not result in flashover problems.
The space within the housing of the transformer is limited. Therefore, it is desirable to provide a high voltage switch which is compact in size yet will maintain the required isolation between high voltage components and grounded components.
Since interruption of a high voltage circuit can cause severe problems if the resulting arc persists for an extended length of time, it is important to attempt to extinguish the arc as rapidly as possible. One method for doing this is to insure that the contacts will be below the oil level within the transformer housing whenever the switch is opened. The oil provides cooling of the arc and other properties well known in the art to provide for rapid arc extinction.
If the oil level within the transformer housing should fall below the level of the high voltage disconnect switch contacts, opening of these switch contacts at this time will cause an arc to be drawn within the vapor-filled region above the oil level. This in turn can result in a catastrophic failure of the transformer due to ignition of the oil vapor by the arc. Therefore, it would be desirable to provide means for detecting a low level of oil within the transformer housing and automatically opening the switch contacts before the oil can drop below the level of the contacts.